The invention relates, in general to processes for recycling thermoplastic material. This invention also relates to processes for rendering scrap thermoplastic suitable for reprocessing and reuse. More particularly, the present invention relates to a method for removing deleterious surfaces such as paint, UV oxidation, etc. from particulate thermoplastic material.
Plastic materials of all types have found many uses in a large number of consumer products. However, such widespread use of plastics has created problems relating to the disposal of plastic consumer products after their useful life. In addition to used consumer products, waste parts, bad parts, scrap, runners, spurs, purgings, etc. generated from plastic manufacturing processes must also be disposed of in an appropriate manner.
Currently, plastic materials are incinerated or buried in landfills. A small percentage of the plastic materials produced are inherently biodegradable in landfills or rendered so through manufacturing modifications. Currently, only a small portion of plastic materials are recycled. Incineration obviously creates a potential environmental problem and is particularly disfavored. Available landfills are reaching capacity and few new landfills are available. This problem is exacerbated as the use of plastic increases and the amount of land available for landfills decreases. With regard to biodegradable plastics, a very low percentage of such plastics are significantly biodegradable. Furthermore, biodegradable plastic materials are extremely costly to manufacture and employ.
Various plastics recycling processes have been proposed, including the process disclosed in U.S. Pat. No. 5,424,013 to Lieberman. In the Lieberman process, thermoplastic to be recycled is ground into particulate form, tested and analyzed and provided with appropriate formulation enhancers and modifiers to adjust any differences in the properties of the recycled material to those properties generally found in virgin thermoplastic. In the Lieberman process, defective thermoplastic parts which have been painted, for example, auto parts and the like, can be granulated and extruded through a devolatilizing extruder. Paint specks remain visible in the processed specimen. Sufficient concentrations of very small paint specks in parts made of reclaimed plastic can give a flawed appearance, and potentially, reduce important physical properties of the plastic material, thereby adversely affecting the overall performance of the finished part.
Various attempts have been made to separate deleterious surface coatings such as paint from polymeric substrate. Processes such as the one disclosed in U.S. Pat. No. 5,340,839 to Gillette are directed toward a method in which articles composed of essentially homogeneous polycarbonate material coated with a scratch-resistant coating are treated with moderate agitation in a solution containing a water-miscible alcohol. The treatment solution employed also contains between about 5% and about 40% alkali metal hydroxides. The thermoplastic material is exposed to the alkaline solution for a period of approximately 20 to 60 minutes with sufficient agitation to provide contact with the surface treated thermoplastic substrate. It is believed that the water-miscible alcohol acts as a co-solvent with the water to penetrate the scratch-resistant surface treatment coating.
U.S. Pat. No. 5,578,135 issued to Lohr et al. discloses a chemo-mechanical stripping of paint from plastic parts. This reference wets the plastic chips with a stripping agent consisting of an anhydrous alkalified glycol, but in no way suspends the chips in the chemical(s). The chips and chemical(s) are then tumbled in a free-fall mixer, a positive mixer, or a rotary mixer to strip the softened/swelled paint from the. The reference also states that it is preferred to include stainless steel abrasive particles in the mixer with the chips.
As can readily be appreciated, extensive exposure to caustic solutions with or without the organic co-solvent can have deleterious effects on various types of underlying thermoplastic substrate materials. Thus, it is desirable, and an object of the present invention, to provide a method for removing undesirable deleterious surface material from various polymeric regrind particles in a manner which is both efficient and effective. Furthermore, it is desirable that the method for removing deleterious surface material from polymeric regrind particles be one which will deter redeposition of the removed particles on the surface of the regrind particles. Furthermore, it is desirable that the process for removing deleterious surfaces from regrind particles be one which permits the use of both mechanical and chemical action to effect removal of the deleterious surface to provide thermoplastic material which can be successfully employed in various post treatment processes. Finally, it is also desirable to provide a method which permits processing with a suitable compatibilizing agent, as necessary, to provide a reclaim thermoplastic material having suitable properties to act as a substitute for virgin material in various applications.